Smart battery separators

ABSTRACT

A separator for an energy storage cell that is provided by a microporous web that includes an irreversible porosity-controlling agent a method for changing an operating characteristic of an energy storage cell.

RELATED APPLICATIONS

This application claims priority to provisional application Ser. No.61/087,197, filed Aug. 8, 2008 and is related to application Ser. No.11/938,327, filed Nov. 12, 2007, now pending.

TECHNICAL FIELD

The disclosure relates to improved energy storage cell separators, andin particular to microporous separators for use in energy storage cellswhich have characteristics that enable enhanced performance undervariable operating conditions.

BACKGROUND AND SUMMARY

In the late 1970s, a no water addition lead-acid battery was developedthat unlike its predecessor, the Gel battery, used a unique veryabsorbent separator to hold the battery acid like a sponge. Theseparator for such an energy storage cell is an absorbent glass mat(AGM) which is a non-woven separator made from spun-glass microfibers.The AGM separator is typically operated in a partially saturated statewhere acid is absorbed by the separator but the total porosity of theseparator is not completely filled with acid. A rechargeable lead-acidenergy storage cell containing the AGM separator operates on theprinciple of oxygen recombination whereby oxygen generated at thepositive plate diffuses through the partially saturated AGM separatorand is therefore able to be chemically reduced at the surface of thenegative electrode to be re-formed back into water. To better facilitatethe oxygen recombination reaction and reduce overall water-loss, theenergy storage cell also incorporates a pressure relief valve thatmaintains a low head-pressure (typically from about 1-5 psi) in thecell.

Accordingly oxygen recombination is used to eliminate water additionduring the life of the energy storage cell. Because the AGM energystorage cells do not “gas freely” they may also be known as valveregulated lead-acid (VRLA) batteries using AGM technology. As statedabove, an AGM separator is a non-woven micro-glass mat separator that issoft, compressible, and very absorbent. In a manner similar to adisposable baby diaper, the AGM separator absorbs and holds the acid.The separator is 92-96% porous and actually absorbs 7-8 times its weightin acid. The AGM energy storage cell is designed so that the thicknessof the plates and the AGM separators fit into the cell case verytightly. In fact, most AGM energy storage cells are designed in such away that the AGM separators are ultimately compressed 20-30% of theiruncompressed volume when stuffed into the cell case. Compression of theAGM separator gives the cell the needed plate-to-separator interfacialcontact and makes the energy storage cell substantially vibrationresistant. The highly porous nature of AGM separators leads to lowerinternal cell resistance and better high rate performance of the energystorage cell.

In most if not all cases, an AGM energy storage cell is a deep cyclebattery that can be used in UPS systems, wheel chairs, portable tools,consumer electronics, alarms, boats, heavy equipment and some toys.Other applications may include emergency lighting, telecommunicationsequipment, backup power systems and solar powered battery systems.

A problem with AGM energy storage cells is that as the cell ages, thecell loses water and the separator dries out. As the separator driesout, the oxygen recombination rate increases and energy storage cellruns hotter. Under certain conditions, the oxygen recombination in theAGM energy storage cell may become too vigorous causing the energystorage cell to go into thermal runaway. If the charger used forcharging the energy storage cell is not temperature compensated, theenergy storage cell may eventually melt and, in severe cases, ignite orburn.

Despite the advances made in the art with respect to separators forenergy storage cells, there continues to be a need for separators forenergy storage cells which exhibit improved physical and electrochemicalproperties over conventional separators. For example, there is a needfor attenuating the oxygen recombination process in an AGM energystorage cell as the cell ages in order to prevent thermal runaway and/ordamage to the energy storage cell.

With regard to the above, one embodiment of the disclosure provides aseparator for an energy storage cell that is provided by a microporousweb that includes an irreversible porosity-controlling agent

Another embodiment provides a method for changing an operatingcharacteristic of an energy storage cell. The method includes applyingfrom about 5 to about 50 weight percent of an irreversible porositycontrolling agent to a separator material. An improved separator may beformed from the separator materials and the irreversibleporosity-controlling agent. The porosity-controlling agent may beselected from agents that change size as a function of temperature,agents that change size as a function of pH, agents that change size asa function of pressure, and agents that change size as a function oftemperature, pH, and/or pressure. The energy storage cell is thenoperated with the separator.

The separators according to the invention exhibit improved properties ascompared to conventional separators. Another advantage of the disclosedembodiments is that the separators may take an active rather thanpassive role in improving the performance of energy storage cells undervariable conditions. Until now, energy storage cell separators have beena passive component of the cells, with the exception of the tri-layerthermal shutdown separator used in the lithium-ion battery industry (seeU.S. Pat. No. 5,952,120 and others assigned to Celgard). The “shutdown”separator is a three-layer structure of stretchedpolypropylene/polyethylene/polypropylene. The internal layer of PE isdesigned to melt at high temperatures thus increasing the electricalresistance of the storage cell and “shutting down” the energy storagecell. The process of “shutting down” the energy storage cell isirreversible and once this occurs the energy storage cell isnon-functional and must be replaced. By comparison, the separatorsdescribed herein may be used to attenuate the electrical resistance ofthe separator as the temperature of the energy storage cell risesthereby enabling continued use of the storage cell even as the cell agesand the separator dries out.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the exemplary embodiments may become apparent byreference to the detailed description of the exemplary embodiments whenconsidered in conjunction with the following drawing illustrating one ormore non-limiting aspects of thereof:

FIG. 1 is a schematic cross-sectional representation of a prior artseparator;

FIG. 2 is a schematic cross-sectional representation of a separatorincluding a porosity controlling agent at a first operating condition;

FIG. 3 is a schematic cross-sectional representation of a separatorincluding a porosity controlling agent at a second operating condition;

FIG. 4 is a graphical illustration of electrical resistances ofseparators containing porosity controlling agents before and afterheating the separators; and

FIG. 5 is a schematic cross-sectional representation of an energystorage cell containing a separator according to embodiments of thedisclosure.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

Separators as described herein may be made of a wide variety ofmaterials including natural and synthetic rubbers, polyolefins,non-woven glass fibers, and the like. Particularly suitable materialsfor AGM separators according to the disclosure are non-woven glassfibers, and non-woven polyolefin separators.

One component of a separator according to the disclosure is aparticulate filler. The particulate filler may be selected from carbonblack, diatomaceous earth and silica particles. A suitable source ofsilica particles is precipitated silica, available from PPG Industries,Inc. of Pittsburgh, Pa., Rhodia Silica Systems of Lyon, France, andEvonik-Degussa GmbH of Dusseldorf, Germany.

Separators made of such as non-woven fiber separators may benefit fromthe inclusion of irreversible porosity-controlling agent as describedbelow.

Suitable porosity-controlling agents that may be used according toexemplary embodiment of the disclosure may be selected from agents thatexpand in response to rising temperatures and contract in response topressure. Other porosity-controlling agents that may be used may expandupon an increase in electrolyte pH or a decrease in pressure. Stillother porosity-controlling agents may expand in response to acombination of one or more of temperature, pressure, and electrolyte pH

Accordingly, the porosity-controlling agents are used in an amountsufficient to affect the porosity of the separator as the separatorages. For example, as the temperature of the energy storage cellincreases, the porosity-controlling agent expands and the separatorporosity decreases.

Preparation of expandable particulate that may be used as aporosity-controlling agent in an AGM energy storage cell is normallyaccomplished by suspension polymerization. A general description of someof the techniques that may be employed and a detailed description ofvarious compositions that may useful as expandable particulate may befound in U.S. Pat. No. 3,615,972. A further description of compositionsuseful as expandable particulate in embodiments of the disclosure may befound in U.S. Pat. No. 4,483,889. Both patents are incorporated hereinby reference.

Examples of commercially available expandable hollow polymericmicrospheres useful in the disclosed embodiments include those made ofpoly(vinylidene chloride-co-acrylonitrile) such as the polymericmicrospheres available from Akzo Nobel, Sweden under the trade nameEXPANCEL. Other commercially available materials having similarconstructions, and comprising, for example, a shell ofmethacrylonitrile-acrylonitrile copolymer, available from MatsumotoYushi-Seiyaku Co., Ltd, Japan under the trade name MICROPEARLmicrobubbles, are also useful as expandable particulate in the presentinvention.

The porosity of a separator may be better understood by referring toFIGS. 1-3. FIG. 1 is a schematic representation of a portion of a priorart separator 10. The separator 10 may include non-woven fibers 12 thatare bonded to one another in a conventional web making process. Forexample, glass fiber separators and polyolefin fiber separators havebeen produced commercially by wet processes on paper making equipmentincluding fourdrinier machines and rotoformers, inclined fourdriniermachines and extended wire rotoformers. In the production of separatormade of glass fibers for VRLA batteries, it is preferred that no organicbinder be added to a furnish from which separator sheets are made; theentanglement of individual micro-glass fibers serves to maintain thesheet in a cohesive structure, and water glass or any of various sulfatesalts, which sometimes form on the fiber surfaces, serves as a binder.Organic binders, however, tend to decrease the ability of a separator towick acid, and to decrease the amount of acid a separator can hold. Dryprocesses for making fibrous energy storage cell separators may also beused as disclosed in U.S. Pat. No. 6,306,539. Void spaces 14 between thefibers 12 provide up to about 95 percent porosity of the separator 10.

Embodiments of the disclosure may also be applicable to other types ofenergy storage cell separators other than fibrous separators. Forexample, porosity controlling particles as described herein may also beincorporated in flexible natural rubber separators such as theseparators made according to U.S. Pat. No. 4,213,815.

In FIGS. 2 and 3, porosity controlling particles 20 are included in afibrous separator 16. The porosity controlling particles 20 may have afirst size indicated by the particles 20A in FIG. 2, and a second sizeindicated by the particles 20B in FIG. 3. In FIG. 2, the particles 20Amay be in an unexpanded or contracted state thereby enabling theseparator 16 to have a first porosity that is similar to the porosity ofthe separator 10 in FIG. 1. In FIG. 3, the particles 20B are in expandedstate thereby decreasing the porosity of the separator 16. Accordingly,the porosity of the separator in FIG. 3 is less than the porosity of theseparators in FIGS. 1 and 2.

The porosity controlling particles included in the separator 16 may beselected from particles that change size in response to temperature, pH,pressure, or a combination of two or more of temperature, pH, andpressure. By selecting a porosity-controlling agent that expands as thetemperature of the energy storage cell increase, the porosity of theseparator 16 may be decreased as illustrated by expansion of particles20A in FIG. 2 to the particles 20B in FIG. 3. As the porosity of theseparator 16 is decreased, the electrical resistance of the separator 16is increased thereby reducing the rate of oxygen recombination which maylayer the operating temperature of the energy storage cell.

The following non-limiting examples are provided to further illustrateone or more aspects of the exemplary embodiments.

Example 1

In the following example, AGM separators were made by tearing 3.4 gramsAGM sheets (˜4.5″×4.5″) having a density of 300 g/m² into pieces by handand mixing the torn pieces into 800 ml of cold water for 24 hours. Amagnetic stirring was used to stir the mixture and to produce a slurry.EXPANCEL microspheres (0.051 grams, 15 wt. %) were then were added tothe cold water and AGM pieces and the mixture was stirred for ˜1 hour.The resulting mixture was poured into a ˜4″ diameter vacuum filterfunnel to form a new AGM sheet. The AGM sheet was vacuum dewatered. Theresulting 4″ round EXPANCEL loaded AGM sheet was then removed from thefilter funnel, placed between sheets of paper towel and blotted dry.Each sheet was then permitted to dry in air for 48 hours. A controlsample AGM sheet was also made by this method but without the additionof EXPANCEL microspheres.

A ceramic hot plate was adjusted to the approximate temperature. A stackof five pieces of 5″×5″×0.25″ safety glass was then placed onto the hotplate and permitted to thermally equilibrate. A laser temperature gunwas then used to determine the temperature of the hot plate beneath thestack of glass plates. When the temperature was properly adjusted theAGM sample was quickly placed beneath the stack of glass plates and adigital timer started. All AGM samples were heat-treated by this methodfor 3 minutes. At the end of 3 minutes the samples were quickly removed.It was noted that at 133° C. and 150° C. that the samples becamenoticeably thicker as the EXPANCEL microspheres expanded.

Electrical resistance for each of the separator webs was determinedbefore and after heating the separator webs. The results are shown inTable 1. In the table, “T-Start” is the temperature as which swelling ofthe EXPANCEL material begins, and “T-Max” is the temperature at whichthe EXPANCEL material reaches its maximum size. In the table AGM EX-1included 15 wt. % of EXPANCEL 051 DU 40 microspheres added to the glassfibers prior to forming the separator. AGM EX-2 included 15 wt. % ofEXPANCEL 920 DU 40 microspheres added to the glass fibers before formingthe separator. The control samples contained no microspheres.

TABLE 1 Electrical Resistance (milli-ohm-cm²) EXPANCEL EXPANCEL Heated(3 min) T -Start (° C.) T- Max (° C.) Unheated 75° C. 130° C. 150° C.AGM Control 215 — — 204 AGM EX-1 110 145 180 413 1,116 793 AGM EX-2 135175 310 471 1,374 1,561

As shown by the foregoing examples, heating the separators containingthe porosity controlling agents (AGM EX-1 and AGM EX-2) provided asubstantial increase in electrical resistance over the AGM controlsamples. The samples containing EX-2 microspheres as compared to thesamples containing EX-1 microspheres had about a 14% increase in theelectrical resistance of the separator when the separated was heated for3 minutes to 75° C. while it provided about a 23% increase at 130° C.and about a 97% increase at to 150° C.

Example 2

In the following example, flexible rubber separators were made with andwithout the porosity controlling agents described above. Two differentEXPANCEL products were used in this example EX-1 and EX-2 as describedabove. Each of the products was added to the flexible rubber separatorduring compounding of the rubber for the separator at the rate of 10 and20 pounds per hundred pounds of rubber. Electrical resistances(milli-ohms-cm²) of a control example and the separators containing theporosity controlling agents upon heating for two minutes are given inthe following table. In FIG. 4, the separators were heated at theindicated temperatures for two minutes.

TABLE 2 Control EXPANCEL 1 EXPANCEL 1 EXPANCEL 2 EXPANCEL 2 Temperature(no EXPANCEL 1) 10 PHR 20 PHR 10 PHR 20 PHR Unheated 21.1 22.8 36.1 21.123.3 135° C. 25.5 27.6 51.8 27.0 31.3 150° C. 29.1 29.4 62.8 29.2 36.7170° C. 40.6 41.8 93.8 39.7 48.6

In the foregoing examples, there was a slight increase in electricalresistance between the control sample and the EXPANCEL 2 sample uponheating for two minutes and containing 10 to 20 phr in the rubber.However, there was a dramatic increase in the electrical resistance forthe EXPANCEL 1 samples at 20 phr after heating at two minutes.Accordingly, it is believed that a combination of two or more types ofporosity-controlling agents may be added to a separator to provide anincrease in electrical resistance at a desired temperature. In otherwords, a separator may be optimized for operation at a desiredelectrical resistance for a particular operating temperature by usingdifferent porosity controlling agents at different loadings in theseparator. Hence, as an example, a flexible rubber separator may contain10 phr of EXPANCEL 1 and 10 phr of EXPANCEL 2 to obtain an electricalresistance that is greater than 27.6 and less than 93.8milli-ohm-cm²/mil

The foregoing separators may be used in an AGM energy storage cell asillustrated in FIG. 5. FIG. 5 is a schematic cross-sectional view of anenergy storage cell 50 according to embodiments of the disclosure. Thecell 50 includes positive and negative terminals 52 and 54 extendingthrough a case 56. The case 56 encloses positive and negative electrodes60 and 62 and separators 64 between adjacent electrodes 60 and 62. Anelectrolyte for the energy storage cell is absorbed within theseparators 64. The separators 64 may include one or more of the porositycontrolling agents described above to increase or decrease theelectrical resistance between the electrodes 60 and 62 by reversiblyincreasing or decreasing the porosity through the separator 64.

At numerous places throughout this specification, reference has beenmade to a number of U.S. Patents and publications. All such citeddocuments are expressly incorporated in full into this disclosure as iffully set forth herein.

The foregoing embodiments are susceptible to considerable variation inits practice. Accordingly, the embodiments are not intended to belimited to the specific exemplifications set forth hereinabove. Rather,the foregoing embodiments are within the spirit and scope of theappended claims, including the equivalents thereof available as a matterof law.

The patentees do not intend to dedicate any disclosed embodiments to thepublic, and to the extent any disclosed modifications or alterations maynot literally fall within the scope of the claims, they are consideredto be part hereof under the doctrine of equivalents.

What is claimed is:
 1. A separator for an energy storage cell comprisinga microporous web wherein the web further comprises an irreversibleporosity-controlling agent.
 2. The separator of claim 1, wherein themicroporous web comprises a non-woven fibrous matrix.
 3. The separatorof claim 1, wherein the microporous web comprises a flexible rubberseparator.
 4. The separator of claim 1, wherein the microporous webcomprises a web made from a mixture of a polymer and silica.
 5. Anenergy storage cell comprising the separator of claim 1, wherein theenergy storage cell is selected from primary and secondary energystorage cells.
 6. The separator of claim 1, wherein theporosity-controlling agent is selected from the group consisting ofagents that change size as a function of temperature, agents that changesize as a function of pH, agents that change size as a function ofpressure, and agents that change size as a function of temperature, pH,and/or pressure to provide a change in an overall porosity of theseparator.
 7. The separator of claim 1, wherein the irreversibleporosity-controlling agent comprises fluid-filled microspheres.
 8. Theseparator of claim 7, wherein the separator comprises from about 5 toabout 50 percent by weight of the fluid-filled microspheres.
 9. Theseparator of claim 1, wherein the non-woven fibrous matrix comprises aglass mat fibrous matrix.
 10. The separator of claim 1, wherein thenon-woven fibrous matrix comprises a polyolefin fibrous matrix.
 11. Theseparator of claim 1, wherein the irreversible porosity-controllingagent comprises microspheres derived from the group consisting ofpoly(vinylidene chloride-co-acrylonitrile) materials andmethacrylonitrile-acrylonitrile materials.
 12. A method for changing anoperating characteristic of an energy storage cell, comprising: applyingfrom about 5 to about 50 weight percent of an irreversible porositycontrolling agent to a separator material; forming a separator from theseparator material and porosity-controlling agent; and operating theenergy storage cell with the separator, wherein the porosity-controllingagent is selected from the group consisting of agents that change sizeas a function of temperature, agents that change size as a function ofpH, agents that change size as a function of pressure, and agents thatchange size as a function of temperature, pH, and/or pressure.
 13. Themethod of claim 12, wherein the operating characteristic is selectedfrom the group consisting of an electrical resistance of the separator,and an overall porosity of the separator.
 14. The method of claim 12wherein the energy storage cell is selected from the group consisting ofprimary and secondary batteries.